There are three known influenza-type viruses which affect human beings: Influenza A, B and C. Influenza A viruses have been isolated from many animal species in addition to humans, while the influenza B and C viruses infect mainly humans. The influenza viruses are enveloped viruses containing negative single-stranded RNA's which are segmented and encapsidated. The influenza virus envelope is characterized by the presence of two surface glycoproteins: hemagglutinin and neuraminidase. The influenza A and B virions are pleomorphic and are usually 80-120 nm in diameter. The influenza C virion has many distinctive properties and is thus distinguished from the closely related A and B virions. Infection with influenza A or B often can cause a highly contagious, acute respiratory illness.
Influenza viruses have a major impact on morbidity leading to increases in hospitalization and in visits to health care providers. High rates of hospitalization are observed for patients over 65 years of age and also for children less than 5 years of age. Influenza virus is also unique among respiratory viruses in being a cause of excess mortality. Furthermore, the spread of influenza virus through a population can result in epidemics which have considerable economic impact. For example, high rates of mortality were observed due to influenza infection during the influenza epidemics of 1957, 1968 and 1977. Fields Viroloqy, Second Edition, Volume 1, pp. 1075-1152 (1990).
There are relatively few known compounds that have significant anti-viral activity against influenza viruses. Two of these, amantadine and rimantadine are approved in the United States for the treatment of influenza virus disease. Both compounds are most effective when used prophylactically and influenza viruses develop resistance to both compounds rapidly. See U.S. Pat. No. 3,152,180 and 3,352,912. Other compounds reported to have activity against influenza viruses are disclosed in U.S. Pat. Nos. 3,483,254, 3,496,228, 3,538,160, 3,534,084 and 3,592,934.
Insofar as is known, pyridazine derivatives have not been previously reported as being useful for the treatment of influenza infection.